The present invention relates to improved green strength of synthetic elastomers. More specifically, the present invention relates to improving green strength of synthetic elastomers through the use of dihydrazide compounds which cross-link the elastomers.
Science and technology in the elastomer field has improved to such an extent that synthetic elastomers have supplemented or replaced natural rubber to a great extent in the fabrication of tires and other rubber products. Stereo-specific polymers and particularly synthetic cis-1,4-polyisoprene have demonstrated physical properties similar to and thus are capable of becoming a complete replacement of natural rubber. However, a major deficiency of rubber elastomers including synthetic cis-1,4-polyisoprene is its lack of sufficient green strength and tack required for satisfactory processing or building properties required as in building tires. The abatement of this deficiency has long been sought by the art and would greatly facilitate in the replacement of natural rubber which is solely produced in tropical climates.
The term "green strength", while being commonly employed and generally understood by persons skilled in the rubber industry, is nevertheless a difficult property to precisely define. Basically, green strength may be defined as the tensile strength developed when an unvulcanized polymer composition of proper configuration is stressed under controlled conditions. Beyond an initial yield point, natural rubber compositions will show increasing strength against rupture while synthetic polyisoprene will fall below the yield point or will increase only slightly above it. In certain practical applications such as uncured tires, belting, shoes and a number of other products in the course of manufacture, green strength is important in assuring the integrity and cohesiveness of the assembly or of parts between building or assembly and ultimate vulcanization. Green strength manifests itself secondarily in the tack or adhesiveness imparted to various formulations employed in the manufacture of a number of rubber articles such as tires, belting, etc. Other things being equal, a higher green strength polymer will yield higher tack or adhesion and will ease the problems of handling a raw product between assembly or building and ultimate vulcanization.
Among the various additive compounds or agents which have been utilized to improve green strength of synthetic rubber elastomers are numerous nitroso compounds as set forth in U.S. Pat. Nos. 2,457,331; 2,477,015; 2,518,576; 2,526,504; 2,540,596; 2,690,780; and 3,093,614. Additionally, various dioxine compounds have been utilized such as those set forth in U.S. Pat. Nos. 2,969,341; 3,037,954, 3,160,595; and British Pat. No. 896,309. Yet another class of additives or compounds are the diesters of 5-norbonene as set forth in U.S. Pat. Nos. 3,817,883 and 3,843,613.
Although numerous compounds have been utilized as additives as by mixing, blending, or added directly to the elastomer chain, or otherwise utilized, the prior art patents are not pertinent in that they do not relate or suggest the use of a dihydrazide compound to cross-link the elastomers.